I've never understood the concern over sustain. If every fret was a dead spot that went "plunk", that would be a problem, but it's a bass, not a guitar. How much sustain do we need?

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^This. And, all other things being equal, no, there won't be any real difference. My aluminum-necked Kramer Duke (30" scale) out sustains every other bass I own (16 of them) with 1 exception; my Kramer 450B (34"). Which, by an amazing coincidence, also has an aluminum neck. And personally, both of them have way too much sustain; but, Ahh, the tone...

I want to get a Jaco style or Marcus Miller style fretless Jazz that sings. (Using DR Fatbeam strings).Have you ever heard MM play a ballad w/his fretless. In jazz soloing, sustain helps a lot in slurring, not each note has to be plucked.

I also want a second fretless (using nylon tape strings) that has a lot of the nice qualities an upright bass has. To get that type of hybrid one has to shoot to sound as close to a DB as possible.

I recently posted that in another thread & got all sorts of interesting advise. The most consistent is that the type of decay of an upright can be approximated by using a short 30" scale (e.g., Hofner, Kala 21-23" scale) & some felt at the bridge.

Using deductive reasoning the converse should be true: a 30" -32" inch bass should have less sustain.
I am thinking about getting SX 30" P & J basses. They are supposed to be quite the bang for the buck

Tune down a half step and put a capo on the 1nd fret of a 34". Loosen the truss rod a little bit to compensate for the decreased string tension and to get the exact same neck curve you're used to. Try to hear the difference.